If you want unique ideas, please read my previous post. I wrote there suggestions on how you can make homemade costumes (illustration below). I also added ways to use "old" costumes (like Cinderella, Snow White, Mickey Mouse) for Nutrition Week Programs.
Come as a cellphone. Yup, you heard that right. Cutout a drawing of the front of an iphone. Make sure it's big enough to cover the front of your body. Then cutout an illustration of the backside of an iphone. Attach them with garter or string so you can wear the costume over your shoulders. Hard to describe -- just look at the illustration below.
Draw fruit slices on a cartolina (white then color according to the fruit that you chose or cut out using colored paper). Attach the "fruit slices" on a skirt. Best fruit for this is orange (see drawing below). You can also try tomato.
You can use balloons to make a bunch of grapes costume. Just blow several small balloons and attach them together to resemble a bunch of grapes.
Lastly, you can also come as a "fruit ninja". If you're familiar with the game then you know what to do. Let your child wear a ninja costume and let him carry s fruit sliced in half.